Pattern, I didn't bother to address the viability question, but unless there is a massive technological breakthrough we're not going to see another commercial SST. The costs and fuel burn of an SST compared to a conventional subsonic airliner make the potential number of paying passengers too small for it to be economically viable. There simply are not that many people who are willing and able to pay a massive price premium to save a couple hours of flight time. No matter how efficient the engines and the airframe, fuel burn is always going to be much higher going supersonic (as one of my college professors put it, 'it takes a lot of energy to break windows ten miles below'), and the stresses of supersonic flight mean high maintenance costs.
The one possibility for a future supersonic passenger aircraft is for a (relatively) small biz jet. Something targeted for the super rich who are willing and able to pay a huge premium to save a few hours (I'm talking about the sort of people who have a 747 as their private jet). The business case would have to assume a small production run (less than 100 aircraft) meaning the massive nonrecurring development and certification costs would need to be spread over a correspondingly small number of sales. On the plus side, the biz jet regulations are somewhat more forgiving than those for large commercial aircraft (i.e. Part 25).